I present a collection of forecasting, analysis, and instrumentation work related to the science cases and detector development for an upcoming camera for the 10-meter South Pole Telescope (SPT), called SPT-3G+. SPT-3G+ is a high-frequency, high-sensitivity camera designed to observe the cosmic microwave background (CMB) at 220, 285, and 345 GHz, targeting a variety of astrophysical and cosmological science cases. I first present forecasts of the expected sensitivity of SPT-3G+ to the recombination-era Rayleigh scattering of the CMB, a signal which probes the expansion and ionization history of the universe just after recombination, and whose first detection is sought by SPT-3G+. I find that, in combination with SPT-3G and Planck data, the expected detection significance is about 1.6-sigma, and that the cosmic infrared background, or CIB, is the major foreground inhibiting a higher detection significance. I next present an attempt to characterize the CIB using data from the 2019-2020 SPT-3G observing seasons, employing an existing physically-motivated model that relates CIB emission to an underlying star-formation rate. Instead of the CIB autospectrum, I fit the CIB x CMB lensing spectrum, which is less susceptible to systematic bias from contaminants such as galactic dust. Finally I present my work on the design, fabrication, and laboratory characterization of feedhorn-coupled direct absorbing 220 GHz microwave kinetic inductance detectors (MKIDs) for SPT-3G+. The detectors perform well from both a microwave and an optical perspective. Resonances are of the intended shape and quality, and are located within the intended microwave readout bandwidth. The detectors show background-dominated performance under a representative optical load, indicating that they possess the required sensitivity for use on the SPT-3G+ focal plane. They show an optical efficiency of approximately 70%. I have begun scaling up to triangular submodule fabrication, and will discuss future plans for the optical testing of deployment-scale submodules.